At present, a variety of user terminal devices are pervasive. They can serve as PDA (Personal Digital Assistant) to support a variety of users, and communicate data processed thereby to the outside using a wireless communication function such as PHS (Personal Handy-phone System: a registered trademark). Now, a prior art example of such a data processing terminal will be described with reference to drawings.
FIG. 1 illustrates data processing terminal 10 as a first prior art example. Data processing terminal 10 comprises a hollow housing (not shown) which contains parent board 11, child board 15, and connector 14 for connecting both boards. Parent board 11 is mounted with a variety of data processing circuits (not shown) comprised of integrated circuits, while child board 15 is mounted with a memory circuit (not shown) such as RAM (Random Access Memory), flash memory and the like.
A card slot (not shown) is formed at one end of the housing, such that separate card-shaped radio communication unit 12 is removably plugged into the card slot. Radio communication unit 12 comprises a built-in radio communication circuit (not shown), and rod-like antenna 13 attached thereto.
Data processing terminal 10 has a user interface (not shown) such as a touch panel, a keyboard and the like on the surface of the housing, such that the data processing circuit on parent board 11 executes a variety of data processing in response to data entered through the user interface and the like.
In data processing terminal 10, as card-shaped radio communication unit 12 is plugged into the card slot of the housing, the radio communication circuit in radio communication unit 12 can be in wired communication with the data processing circuit on parent board 11, permitting data processing terminal to make radio communications in a frequency band near 1.9 (GHz) using a PHS function through radio communication unit 12.
In other words, data processing terminal 10 can make radio communications with the outside as required with the aid of radio communication unit 12 to transmit data processed by the data processing circuit over the air, as well as to process data received over the air with the aid of the data processing circuit. Further, when data processed by the data processing circuit is stored in the memory circuit on child board 15, data processing terminal 10 can process a large capacity of data.
In data processing terminal 10 as described above, parent board 11 is substantially entirely formed with a metal-made first ground plane, while child board 15 is likewise substantially entirely formed with a metal-made second ground plane (not shown), wherein both ground planes determine potential references for circuits on the respective boards.
Connector 14, which connects the two boards, has a plurality of signal terminals and a plurality of ground terminals arranged in parallel, wherein one ground terminal is inserted, for example, every three signal terminals. With such a structure, as child board 15 is mounted on parent board 11, data processing circuit on parent board 11 is connected to the memory circuit on child board 15 through the signal terminals of connector 14, and the first ground plane on parent board 11 is connected to the second ground plane on child board 15 through the ground terminals of connector 14 to connect the potential references provided by the ground planes on both boards.
While FIG. 1 illustrates data processing terminal 10 which has child board 15 mounted on parent board 11 through a pair of connectors 14 that are removable in the vertical direction, there is also a product, data processing terminal 20 illustrated in FIG. 2, which is a second prior art example, wherein child board 15 is mounted to and removed from connector 21 in the transverse direction.
Also, as data processing terminal 30 in FIG. 3, which is a third prior art example, there is a product which comprises connection pads 31, 32 formed on the front surface and back surface of parent board 11 and child board 15 and electrically connected to the first ground plane and second ground plane, respectively, with connection pads 31, 32 being electrically connected through tubular metal columns 34 and screws 35 which make up auxiliary connecting means 33. It should be noted that such metal columns 34 and screws 35 are generally intended to mechanically hold child board 15, so that they are disposed near a pair of corners at diagonal positions of child board 15.
In the aforementioned data processing terminal 10, 20, first ground plane 17 of parent board 11 and second ground plane 18 of child board 15 connected through connectors 14, 21 are positioned in parallel with each other, as illustrated in FIG. 4. Also, data processing circuit 19 composed of a multiplicity of electronic parts such as LSIs (Large Scale Integration) and signal wires is mounted on first ground plane 17 of parent board 11.
Since data processing circuit 19 transmits repetitive signals and non-repetitive signals at particular frequencies within the circuit when it processes data, an electromagnetic field is generated in the neighborhood, associated with frequency components and harmonic components of the transmitted signals. This electromagnetic field not only causes a high frequency current to flow into first ground plane 17 of parent board 11 but also induces a high frequency current into second ground plane 18 of child board 15 disposed in the neighborhood.
The inventors found that the ground structure comprised of first ground plane 17 of parent board 11, second ground plane 18 of child board 15, and the ground terminals of connectors 14 as illustrated in FIG. 4 well resembled an antenna element of a quarter wavelength resonant antenna referred to as an “inverted L-shaped antenna” or an “inverted F-shaped antenna”, as illustrated in FIGS. 5A and 5B (reference: “Small Antennas” K. Fujimoto, A. Henderson and J. R. James, Research Studies Press, Chapter 2.4).
In thinking in the foregoing manner, from the fact that connector 14 comprises a plurality of ground terminals, second ground plane 18 of child board 15 has an edge close to connector 14 that corresponds to a short-circuited end of the antenna element, and an edge opposite to connector 14 that corresponds to an open end of the antenna element. If a current induced into second ground plane 18 from data processing circuit 19 includes frequency components which cause second ground plane 18 to generate quarter wavelength resonance, a strong electromagnetic field is generated around second ground plane 18, thereby irradiating strong electromagnetic waves a long way off.
When considering child board 15 that has a memory circuit mounted thereon, child board 15 generally has edges extending over approximately 25 to 75 (mm), so that the quarter wavelength resonance occurs at frequency in a range of approximately 1 to 3 (GHz).
Since conventional data processing circuit 19 internally transmits signals at a basic frequency around several MHz, its harmonics are also on the order of 100 (MHz) at most. Thus, data processing circuit 19 is free from the quarter wavelength resonance because harmonic components are largely lower than the frequency at which the quarter wavelength resonance occurs in the ground structure.
Recently, however, the trend of increasing the processing speed of integrated circuits has increased the basic frequency of data processing circuit 19 to several hundred MHz, and its harmonics have also spread to as high as several GHz. Since the harmonics overlap the frequency at which the aforementioned ground structure is at the quarter wavelength, the ground structure suffers from the resonance.
On the other hand, with the recent development of radio communications, GHz bands are increasingly utilized for radio communications, such as approximately 1.9 (GHz) in PHS; approximately 800 (MHz), approximately 1.5 (GHz), and approximately 2.0 (GHz) in portable telephones; and approximately 2.4 (GHz) in wireless LAN (Local Area Network) and Bluetooth.
From the foregoing background, the resonant frequency associated with the ground structure overlaps the frequency bands used for radio communications, giving rise to a problem that radio communications are impeded by the ground structure.
While it has been predicted from before that the electromagnetic field generated by data processing circuit 19 would directly affect radio communication unit 12, and countermeasures have been taken therefor, nobody has been able to predict that the operation of data processing circuit 19 on parent board 11 causes child board 15 and the like to act as a resonant antenna, and an electromagnetic field generated thereby affects radio communication unit 12.
As illustrated in FIG. 3, in data processing terminal 30 of the third prior art example, first and second ground planes 17, 18 are each short-circuited by auxiliary connecting means 33 at a corner near an open edge thereof, so that the ground structure in this prior art example is free from the quarter wavelength resonance, but can suffer from half wavelength resonance. Since the half wavelength resonance is induced at frequency on the order of 2-6 (GHz) with the aforementioned size, this range of frequencies, though slightly higher than the previous examples, will impede the communications as well because it is close to the frequency bands used for radio communications.
The description made in connection with the aforementioned data processing terminals 10, 20, 30 clarifies that radio communications made by removably connected radio communication unit 12 are impeded by the ground structure within the terminal. Such impeded radio communications will be experienced if radio communication unit 12 is disposed near the ground structure.
Therefore, communication failures can arise even if a radio communication circuit is connected to the data processing terminal through a connection cable, or when it is placed near the data processing terminal, though not connected thereto (not shown).
To solve the problem as mentioned above, child board 15 may be covered with a metal case (not shown), with the metal case being connected to first ground plane 17 of parent substrate 11, to isolate radio communication unit 12 from a strong electromagnetic field generated by child board 15. However, even with the structure using the metal case for fully covering child board 15, the metal case causes a problem of an increase in the size of data processing terminal 10, 20, 30.